Electrical printing apparatus with means to control boundary layer effect



Jan. 16, 1968 w. E. JOHNSON E AL ELECTRICAL PRINTING APPARATUS WITH MEANS TO CONTROL BOUNDARY LAYER EFFECT Filed July 8, 1966 INVENTORS Mil/ c2111 E. Jbh nson 00a Ids/ .152 (76/212 MZZZM WK @AQZafl/ b 97 34.5?

United States Patent ELECTRICAL PRINTING APPARATUS WITH MEANS T0 CONTROL BOUNDARY LAYER EFFECT William E. Johnson, Temperance, and Douglas F. St. John, Perry, Mich., assignors to Owen-Illinois Inc., a corporation of Ohio Filed July 8, 1966, Ser. No. 563,829 1 Claim. ('Cl. 101-1) ABSTRACT OF THE DISCLOSURE Electrical printing apparatus of the type wherein an image of light weight powder particles is electrically transferred across an air gap between the surfaces of a rotating drum and a fiat plate at relatively high speeds of movement of the drum and plate. Air turbulence created in the region of transfer by boundary layers of air moving with the drum and plate surfaces is minimized by nozzles located on opposite sides of the air gap which respectively supply and withdraw air from these regions to counteract the boundary layer air induced turbulence, thereby improving the resolution of the transferred powdered image.

Prior apparatus, whether electrostatic offset method or direct electrostatic method, transfers powdered images at relatively slow speeds eg, 50 ft. per minute. The electrostatic pulse method produces images with a zero relative velocity between the screen-stencil and substrate. At these relatively slow speeds between the surfaces between which the powder is transferred, there is little or no difficulty encountered with air currents generated by the moving surfaces which tend to blow or scatter the powder particles in transit between the surfaces. In order to in crease the production rate, higher speeds of relative movement betwen the various surfaces is required and the effect of the boundary layers of air which travel with the moving surfaces becomes noticeable. Where two moving surfaces are located on opposite sides of the gap across which the powder is transferred, air currents and turbulence in the relatively narrow gap tend to deflect and scatter the powder, giving images of poor resolution.

It is therefore an object of this invention to provide improved methods and apparatus for transferring images of printing powder particles between moving surfaces to achieve minimum scattering and improved resolution of the applied image.

Another object of our invention is to provide improved methods and apparatus for applying images of printing powder particles to the moving surface of an object in which boundary layer air current effects are substantially reduced or eliminated.

Other objects and features of the invention will become apparent by reference to the following specification and to the drawings.

In the drawings:

FIGURE 1 is a schematic diagram embodying features of the invention; 7

FIGURE 2 is a fragmentary schematic diagram of a portion of the diagram shown in FIGURE 1 and on an enlarged scale; and

FIGURE 3 is a fragmentary diagrammatic illustration of a high pressure area disclosed in FIGURES 1 and 2.

Before explaining the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also,

it is to be understood that the phraseology or terminology 3,363,545 Patented Jan. 16, 1968 employed herein is for the purpose of description and not of limitation.

In the drawings, one form of the invention is shown in which a powder image is transferred from the surface of a rotatable drum 10 to a substrate in the form of a fiat plate-like article 14 which is conveyed past the drum by a suitable conveying means such as a belt 15 driven in a conventionalmanner in the direction of the arrow A of FIGURE 1. During operation, drum 10 is driven in rotation in the direction of the arrow B at a speed related to the speed of movement of conveyor 15 such that if the surface of drum 10 were in contact with belt 15, the drum would roll onto the belt with no-slip contactin other words the peripheral speed of the drum is the same as the speed of movement of conveyor belt 15. The moving peripheral surface of drum 10 and the upper surface of sub strate 14 carry with the moving surfaces a boundary layer of air indicated at 16 and 20. At relatively slow speeds, the effect of the moving boundary layers of air is negligible, but when belt 15 and drum 10 are driven at higher speeds, the effect of the boundary layer of air becomes noticeable.

In the disclosed embodiment, the apparatus is employed to electrostatically transfer a powder image such as I from the peripheral surface of drum 10 to the surface of plate or substrate 14. The image I is applied to the surface of drum 10 in the form of a layer of printing powder particles by any of several well known means such as the Xerographic process, for example. Apparatus for applying the powder image to the drum surface is not disclosed for the sake of clarity.

The image is transferred from the drum surface to substrate 14 by establishing an electric field between the drum surface and substrate surface. As these two surfaces move into proximity to each other in the region 24, the electric charge on substrate 14 becomes effective to electrically attract the powder particles from the drum surface to the substrate surface. This transfer occurs across the relatively small air gap in the region 24 at which substrate 14 is at its closest proximity to the surface of drum 10.

In FIGURES 1 and 2, the apparatus is shown at a point of time in which an image is partially transferred to substrate 14, the transferred portion of the image being designated I while the portion of the image remaining on the drum is designated 1 Powder particles are indicated in the region 24 in transit across the air gap between the surface of drum 10 and substrate 14. Because the particles in transit are in the form of a very fine powder having very little mass, they are subject to the influences of air currents such as those generated by boundary layer air flowing with the moving surfaces of the drum and substrate.

At the left-hand side of region 24, the boundary layers of air moving with the drum surface and substrate surface are both directed toward region 24. Because of the curvature of drum 10, the space between the two surfaces steadily decreases as region 24 is approached from the left and the boundary layer of air carried into this region of steadily decreasing volume thus generates a high pressure region in which air currents and turbulence affecting the transfer of particles is present. At higher speeds, the air gap between drum 10 and substrate 14 functions as a restricted passage which is simply not large enough to pass all of the boundary layers of air flowing into it, hence these boundary layers create turbulence within region 24 tending to blow or scatter particles in transit between the drum and substrate.

At the opposite or right-hand side of region 24 as viewed in the drawings, a similar effect is present in reverse in that the moving drum and substrate surfaces create a partial vacuum or low pressure region. The turbulence and air flow induced by moving drum 10 and substrate 14 at relatively high speeds tends to blow or scatter powder particles in transit between the drum and substrate, thus producing images with poor resolution and fuzzzy edges.

To overcome this effect, the present invention provides a pair of nozzles 40 and 44 which respectively supply and withdraw air from the critical regions to produce a smooth and balanced fiow of air around the region 24 to establish within region 24 a condition in which there is a minimum amount of movement of air. Nozzles 40 and 44 are not circular, but are elongated in their direction parallel to the axis of rotation of drum 10 to completely overlap that dimension of the image measured parallel to the axis of drum 10.

Nozzle 44 is connected to a vacuum source V and nozzle 40 is connected to a pressure source P through a regulating valve assembly designated generally 50 whichmay be provided with conventional means, not shown, for adjustably regulating the rate of flow of air through the nozzles. The rates of flow through the respective nozzles are adjusted until, within the region 24, a dead air space is created so that particles in transit between the two surfaces are not blown off course by air currents during transfer.

While one embodiment of the invention has been described in detail, it will be apparent to those skilled in the art that the described embodiment may be modified. Therefore, the foregoing description is to be considered exemplary rather than limiting, and the true scope of the invention is that defined in the following claim.

We claim:

1. An electrical printing apparatus wherein a patterned layer of powder particles is electrically transferred between a cylindrical drum rotatable about its axis and a fiat platelike surface lying in a plane parallel to the drum axis and 3 spaced from the peripheral surface of the drum by a relatively narrow air gap, said particles being transferred across said air gap at the region wherein the minimum spacing exists between the drum periphery and platelike surface while said drum is rotated and said drum and plate are translated relative to each other at speeds such that the boundary layers of air carried by and adjacent the drum and plate surfaces generate air turbulence adjacent to and within said air gap, said printing apparatus including means for inducing a flow of air on each side of said gap comprising a pair of nozzles, each of said nozzles projecting into the space between the surfaces of said drum and said plate-like surface closely adjacent one side of the gap and having a nozzle opening elongated in a direction parallel to the axis of said drum, means for supplying air under pressure to the nozzle at that side of said gap at which the boundary layers of air are moving away from said gap, and means for applying a partial vacuum to the nozzle at that side of said gap at which the boundary layers of air are moving toward said gap.

References Cited UNITED STATES PATENTS 1,730,748 10/1929 Schmidt 230122 2,000,741 5/1935 Buckland 230-122 2,242,182 5/1941 McCann 107-7.1 XR 2,406,918 9/1946 Stalker 24442.41 2,478,726 8/1949 Trey 24442.41 2,709,917 6/1955 Bruynes 230122 2,892,582 6/1959 ORourke 230-95 2,948,148 8/1960 Anfrerille et a1. 230-122 3,081,698 3/1963 Childress et al. 101-129 3,216,455 11/1965 Cornell et al. 230-422 ROBERT E. PULFREY, Primary Examiner.

E. S. BURR, Examiner. 

